I think C ++ is the only high-level language where strings are mutable. This means that replace cannot change the line on which it works, and therefore must return a new line.
Try instead
var element = jQuery('.checkout td h4'); element.html(element.html().replace(":", "."));
Or perhaps more correctly (as you may have multiple elements).
jQuery('.checkout td h4').html( function (index, oldHtml) { return oldHtml.replace(":", "."); } );
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